A nonaqueous lithium secondary battery comprising positive and negative electrodes capable of storing/releasing lithium ion and a nonaqueous electrolytic solution having dissolved therein a lithium salt such as LiPF6 and LiBF4 has been developed and is used in practice.
Various materials have been proposed as the negative electrode material of this battery, but in view of high capacity and excellent flatness of discharge potential, a graphitic carbon material such as natural graphite, artificial graphite obtained by graphitizing coke or the like, graphitized mesophase pitch, and graphitized carbon fiber is used.
Also, an amorphous carbon material is used because this is relatively stable to some electrolytic solutions. In addition, a carbon material imparted with properties of both graphite and amorphous carbon by coating or attaching amorphous carbon on the surface of a graphitic carbon particle is also used.
In Patent Document 1, a spheroidized graphitic carbon material enhanced in the rapid charge-discharge characteristics by applying a mechanical energy treatment to a graphitic carbon particle that is originally in a flake, scale or plate form, to give a damage to the graphitic particle surface and at the same time, spheroidize the particle is used, and it is further proposed to use a spheroidized carbon material having a multilayer structure, which has the properties of graphite and amorphous carbon by virtue of coating or attaching amorphous carbon on the surface of the spheroidized graphitic carbon particle and simultaneously has rapid charging-discharging property.
However, in recent years, the application of a nonaqueous lithium secondary battery is expanding, and a nonaqueous lithium secondary battery having higher rapid charging-discharging property than ever before and at the same time, having high cycle characteristics is demanded in use for electric power tools, electric cars and the like, in addition to conventional use for notebook-size personal computers, mobile communication equipment, portable cameras, potable game machines and the like.
With respect to improvement of the cycle characteristics, for example, Patent Document 2 has proposed a nonaqueous lithium secondary battery using, as negative electrode materials, a carbonaceous material particle having a multilayer structure, where the R value obtained from the Raman spectrum is 0.2 or more, and an amorphous carbonaceous particle having low crystallinity, where the X-ray interplanar spacing d002 is from 3.36 to 3.60 Å. However, this battery has a problem that the irreversible capacity attributable to the amorphous carbon particle is increased, and more improvements are needed on the cycle characteristics and rapid charge-discharge characteristics required of the recent lithium secondary battery.
Also, Patent Document 3 has proposed a negative electrode material composed of a mixture of a coated graphite particle whose surface is covered with amorphous carbon, and a non-coated graphite particle whose surface is not covered with amorphous carbon.
In the description of Patent Document 3, it is stated that the non-coated graphite particle indicates a graphite particle where the ratio [I1360/I1580] of the peak intensity (I1360) near 1,360 cm−1 to the peak intensity (I1580) near 1,580 cm−1 in argon laser Raman spectrometry at a wavelength of 5,145 Å is 0.10 or less. It is also stated that the coated graphite particle indicates a graphite particle where the ratio [I1360/I1580] of the peak intensity (11360) near 1,360 cm−1 to the peak intensity (I1580) near 1,580 cm−1 in argon laser Raman spectrometry at a wavelength of 5,145 Å is from 0.13 to 0.23.